Forty isolates of P. oryzae were obtained from disease outbreak areas in various
regions of Thailand. The fungi were divided into two groups; group 1 included 2 isolates which
cause only neck blast with disease score of 7 – 9 and group 2 included 23 isolates which cause
leaf and neck blast symptoms with disease scores of 5 – 6 and 7 – 9, respectively. The two
groups of fungi were tested on 31 NILs. The avirulence genes including Avr-Pik (Avr-Pik, Avr-
Pikm, Avr-Pikp), Avr-Pi1, Avr-Pi3, Avr-Pi5(t), Avr-Pi7(t), Avr-Pi9, Avr-Pi20, Avr-Pii and Avr-
Pita were found in both groups whereas Avr-Pi11, Avr-Pi12, Avr-Pi19, Avr-Pia, Avr-Pib, Avr-
Pikh, Avr-Piks, Avr-Pish, Avr-Pita, Avr-Piz, Avr-Piz-5 and Avr-Piz-t were detected only in the
group 2. The results explained the relationship between fungi and the avirulence genes
functioned at different stages of rice.